LCD display panels have under gone several evolutions, especially in backlighting technology, since their introduction for use in television and computer monitors.
Cold Cathode Fluorescent Lamps (CCFL): CCFL backlighting was the dominant lighting technology when LCD displays were first mass produced. Generally two CCFLs were placed at opposite edges of the display. A diffuser and two polarizers spread the light out evenly across the back of the display. Unlike white LEDs, CCFLs have an even-white spectral output resulting in better color gamut for the display. CCFLs however are substantially less energy efficient than LEDs and require costly electronics to convert whatever voltage the television or monitor uses (typically 5 or 12 v) to the high voltage needed to light a CCFL. The voltage conversion is normally performed using an inverter. The thickness of the inverter transformer also limits how thin the display can be made.
Edge Lit—White LED: The LCD panel is lit by a row of white LEDs placed at one or more edges of the screen. Like CCFLs, this technology requires a light diffuser to spread the light evenly across the whole display. The color gamut of white LEDs is less than that found in CCFL backlighting.
Array Lit—White LEDs: The LCD panel is lit by a full array of white LEDs placed behind a diffuser behind the LCD panel. LCD displays that use this implementation will usually have the ability to dim the LEDs in the dark areas of the image being displayed, effectively increasing the contrast ratio of the display. As of 2012, this design gets most of its use from LCD televisions. The color gamut of this implementation is the same as that in edge lit with white LEDs.
Array Lit—RGB-LED: This backlighting technology is similar to the Array Lit with white LEDs, except the panel is lit by a full array of RGB LEDs. While displays lit with white LEDs usually have a poorer color gamut than CCFL lit displays, panels lit with RGB LEDs have very wide color gamuts. This implementation is very costly and most popular on professional graphics editing LCD displays.
Technology to improve the color gamut in LCD display panels is also an ongoing effort. CCFL lighting provides better color gamut than LED lighting but has the disadvantage of substantially higher cost. White LEDs cannot provide a pure white light. In an attempt to improve the spectral output of LEDs the lens of a blue light emitting LED is coated with a yellow phosphor which then emits a whiter light. Another approach called remote phosphor is coats the lens of an ultraviolet emitting LED with a phosphor which then emits a whiter light. Other approaches to improve the color gamut of the backlight have not been used in production because of cost or difficulty in manufacturing process.
The color gamut of backlighting technology is a tradeoff between acceptable colors displayed to the viewer and manufacturing process and cost.
Recently a new technology has been developed. This technology is based on Quantum Dots (QD) which is semiconductor nano size crystals that emit light of specific wavelengths when stimulated with light.
This display technology is similar to organic light-emitting diode (OLED) displays. One of the advantages of QDs is that they emit very pure light. For example, the frequency of light emitted from any given QD is dependent on the width of the QD crystal. A QD that is approximately 7 nanometers (nm) in diameter will, when stimulated with blue light, emit red light. Because a group of QDs “tuned” for red light emission will all not be exactly 7 nm, slightly different frequencies or wavelengths centered around what is emitted by a QD that is exactly 7 nm will be emitted. The result is red light emitted with a peak wavelength of what would be emitted by a 7 nm crystal. The light will be, for all intents and purposes, pure red light which will appear to a viewer as a saturated red. For QD crystals that are approximately 3 nm, the emitted light will appear to be a saturated green color.
Quantum dots incorporate the best aspects of both organic light emitters and inorganic light emitters (LEDs). With many promising advantages, QD LED or QLED is considered as a next generation display technology which is still several years off.
Some companies have introduced technology which uses films with red and green quantum dots embedded into the film. This film is then back lit with blue light typically emitted by blue LEDs. The quantum dots fixed to the film will, when stimulated by blue light will in turn emit red and green light and will pass some amount of blue light around the quantum dots as scattered blue light. The resultant white light (red, blue and green) will mix and appear as pure white light at the back of the color film/LCD.
The use of color filter film on the back surface of LCD display panels is well known in the art. These films have colored dots of red, blue and green printed or deposited on the film. The dots are arranged on the film such that when the film is applied to the back of a LCD display panel they are aligned such that each dot is directly over the area where a matching pixel gate is positioned within the LCD panel itself.